1. Field of the Invention
The present invention relates to an optical disc apparatus and a correction servo control signal generation method that can be suitably applied to optical disc apparatus designed to reproduce information from an optical disc having a uniform recording layer where recording marks are formed.
2. Description of the Related Art
Optical discs of the known types such as compact discs (CD), digital versatile discs (DVD) and Blu-ray discs (registered trademark, to be referred to as BD hereinafter) have become very popular. Optical disc apparatus for playing such optical discs are designed to irradiate a desired track to be irradiated with a light beam of the signal recording layer with the light beam and read reflected light coming from the optical disc.
As a light beam is applied onto the signal recording layer of an optical disc by means of such a known type optical disc apparatus, the light beam is reflected by the signal recording layer so that the optical disc apparatus can receive reflected light with not less than a predetermined level of intensity at all times. Therefore, known type optical disc apparatus are designed to perform servo control of generating various error signals (e.g., focus error signal and tracking error signal) that represent the quantity of displacement of the light beam from the desired track according to the received reflected light so as to irradiate a desired position with a light beam.
Additionally, known type optical disc apparatus are designed to record information on an optical disc by applying a light beam onto the signal recording layer of the optical disc so as to modify the local reflectance of the signal recording layer.
It is known that the size of the beam spot that is formed when the light beam emitted from an optical disc apparatus is converged by an objective lens is defined by λ/NA (λ: wavelength of light beam, NA: numerical aperture) and the resolution is proportional to this value. For example, in the case of a BD system, data of about 25 GB can be recorded per layer of an optical disc with a diameter of 120 mm.
Various pieces of information that can be recorded on optical discs include contents such as music contents and video contents and data including those to be used for computers. Particularly, the quantity of information to be recorded on an optical disc has increased due to the availability of high definition images and high sound quality pieces of music and an increased number of contents are required to be recorded on a single optical disc. Thus, there is a demand for optical discs having a larger capacity.
Under these circumstances, techniques have been proposed for simplifying the operation required to record information on and increasing the capacity of an optical disc by recording a standing wave in a uniform recording layer as recording mark typically by means of hologram (refer to, e.g., Jpn. Pat. Appln. Laid-Open Publication No. 2008-071433).
An optical disc apparatus realized by utilizing the technique of the above Patent Document applies a light beam onto an irradiation line that connects the centers of recording marks of an optical disc and receives reflected light coming from the optical disc. Then, the optical disc apparatus detects the presence or absence of a recording mark with the reflected light to reproduce information.